Cardiovascular disease, specifically atherosclerosis, remains a leading cause of death in developed countries. Atherosclerosis is a multifactorial disease that results in a narrowing, or stenosis, of a vessel lumen. Briefly, pathologic inflammatory responses resulting from vascular endothelium injury includes the expression of chemokines and adhesion molecules leading to the migration of monocytes and vascular smooth muscle cells (VSMC) from the sub endothelium into the arterial wall's intimal layer. There the VSMC proliferate and lay down an extracellular matrix causing vascular wall thickening and reduced vessel patency.
Cardiovascular disease caused by stenotic coronary arteries is commonly treated using either coronary artery by-pass graft (CABG) surgery or angioplasty. Angioplasty is a percutaneous procedure wherein a balloon catheter is inserted into the coronary artery and advanced until the vascular stenosis is reached. The balloon is then inflated restoring arterial patency. One angioplasty variation includes arterial stent deployment. Briefly, after arterial patency has been restored, the balloon is deflated and a vascular stent is inserted into the vessel lumen at the stenosis site. After expansion of the stent, the catheter is then removed from the coronary artery and the deployed stent remains implanted to prevent the newly opened artery from constricting spontaneously. An alternative procedure involves stent deployment without prior balloon angioplasty, the expansion of the stent against the arterial wall being sufficient to open the artery, restoring arterial patency. However, balloon catheterization and/or stent deployment can result in vascular injury ultimately leading to VSMC proliferation and neointimal formation within the previously opened artery. This biological process whereby a previously opened artery becomes re-occluded is referred to as restenosis.
Implantable medical devices have become increasingly more common over the last fifty years and have found applications in nearly every branch of medicine. Examples include joint replacements, vascular grafts, heart valves, ocular lenses, pacemakers, vascular stents, urethral stents, and many others. However, regardless of the application, implantable medical devices must be biocompatible. They must be fabricated from materials that will not elicit an adverse biological response such as, but not limited to, inflammation, thrombogensis or necrosis. Thus, early medical devices were generally fabricated from inert materials such as precious metals and ceramics. More recently, stainless steel and other metal alloys have replaced precious metals and polymers are being substituted for ceramics.
Stents and/or drug therapies, either alone or in combination with the PTCA procedure, are often used to avoid or mitigate the effects or occurrence of restenosis. In general, stents are mechanical scaffoldings which may be inserted into a blocked or narrowed region of a passageway to provide and maintain its patency. During implantation, a stent can be positioned on a delivery device (for example and without limitation a balloon catheter) and advanced from an external location to an area of passageway blockage or narrowing within the body of the patient. Once positioned, the delivery device can be actuated to deploy the radially expandable stent. Expansion of the stent can result in the application of force against the internal wall of the passageway, thereby improving the patency of the passageway. Thereafter, the delivery device can be removed from the patient's body.
Stents may be manufactured in a variety of lengths and diameters and from a variety of materials ranging from metallic materials to polymers. Stents may also incorporate and release drugs (i.e., “drug-eluting stents”) that can affect endothelialization as well as the formation of and treatment of existing plaque and/or blood clots. In some instances then, drug-eluting stents can reduce, or in some cases, eliminate, the incidence of endothelialization, thrombosis and/or restenosis.
Additionally, recent advances in in situ drug delivery has led to the development of implantable medical devices specifically designed to provide therapeutic compositions to remote anatomical locations. Perhaps one of the most exciting areas of in situ drug delivery is in the field of intervention cardiology. Vascular occlusions leading to ischemic heart disease are frequently treated using percutaneous transluminal coronary angioplasty (PTCA) whereby a dilation catheter is inserted through a femoral artery incision and directed to the site of the vascular occlusion. The catheter is dilated and the expanding catheter tip (the balloon) opens the occluded artery restoring vascular patency. Generally, a vascular stent is deployed at the treatment site to minimize vascular recoil and restenosis. However, in some cases stent deployment leads to damage to the intimal lining of the artery which may result in vascular smooth muscle cell hyperproliferation and restenosis. When restenosis occurs it is necessary to either re-dilate the artery at the treatment site, or, if that is not possible, a surgical coronary artery bypass procedure must be performed.
Recently, it has been determined that drug-eluting stents coated with anti-proliferative drugs such as, but not limited to, rapamycin and its analogs and paclitaxel have shown great promises in preventing restenosis. However, there is a need to develop additional and potentially more efficacious drug-eluting stents (DES). One critical factor in DES efficacy is the drug elution rate. Drug elution is generally a factor of the drug's solubility in the polymer coating applied to the stent.
One of the critical components of a drug eluting stent is the polymer coating material. The coating material serves as the reservoir from which drug release is controlled. After all of the drug has bee released or depleted, the polymeric coating may serve as a permanent implant material. For the success of a permanent implant, the must be biocompatible. The polymer coating is a foreign material to the body of a patient. It may cause an unwanted immune reaction from the body and result in implant rejection. Thus, there continues to be a need for improving biocompatibility for polymer materials which are used to fashion or coat implantable medical devices.